AN EXPERIMENTAL TEST OF THE HAMILTON-ORIANS HYPOTHESIS FOR THE ORIGINOF AVIAN BROOD PARASITISM

Hamilton and Orians (1965) hypothesized that obligate interspecific br ood parasitism might evolve when nest destruction during laying leads birds to lay subsequent eggs in their clutches in conspecific nests. I t is unknown whether facultative parasitism after nest destruction is common in birds, but it is a reasonable expectation since many nesting species, especially colonial ones, commonly practice intraspecific pa rasitism. Red-winged Blackbird (Agelaius phoeniceus) nests were remove d after females laid their first eggs and other nests in the same colo nies were monitored to determine whether the affected females laid sub sequent eggs parasitically. No parasitic eggs appeared in other nests and, in some cases, fresh eggs were found the next day at the sites of the removed nests. Thus the initial response in the Hamilton-Orians h ypothesis is uncommon or absent in Red-winged Black-birds. A literatur e review and new data indicate that Red-winged Blackbirds lack defense s likely to forestall brood parasitism so the failure to elicit parasi tism experimentally and its rarity or absence in nonmanipulative studi es of this species is enigmatic